WOT RPM too high. Where is the slip?

[muskyfins]

2000 Rinker FV270
5.7 Mercruiser TBI
Bravo 3

Background: for years she always runs exactly 30 mph (SOG GPS speed) at 4000 rpm trimmed full in/down with no tabs, half tank of gas and 2 people. (obviously slight difference with current +/-1 mph) First shakedown run each year with only myself in boat trimmed all the way up 38 mph at 4600 rpm. Like religion.

On the return to our slip yesterday, the engine seemed very free revving so I left it at full throttle after coming on plane to see what would happen and she went all the way to 5000. I backed down to 4000 and was running a little under 25 mph. Verified speed with 2 additional gps-all correct.

I pushed the throttle and again she went almost to 5000 before I backed down. This boat has never been able to rev that high when trimmed up much less when trimmed down. I haven't checked the tach with a test, but I have quite a bit of mechanical experience and will say as she approached 5k, she was screaming. I don't think the tach was off. I usually cruise at 3800-4000.

When we got to our slip, (6 miles) the clutch engaged solidly fore and aft, so I don't think it was that. There was no rubber smell, and my experience with couplers is generally catastrophic (works or nothing). There was no unusual vibration. Seas were calm with little to no wind and I was in the lake with no current.

Is something slipping? It was fine on the way out to our anchor spot, so I'm thinking this isn't a prop problem.

As always, thanks for any and all suggestions.

 

[dingbat]

Sounds like a spun prop

 

[Scott06]

You said returning to the slip - have you verified prop is not damaged and hull is clean and not waterlogged?

 

[muskyfins]

Sounds like a spun prop

Bravo 3 is direct to shaft. No hub.

 

[muskyfins]

You said returning to the slip - have you verified prop is not damaged and hull is clean and not waterlogged?

Boat is kept in a wet slip. This change occurred while we were at anchor for the day.

Bottom is clean and has ablative anti-fouling paint.

I don't recall hitting anything on our way out. No vibration, but I did not trim drive all the way up and inspect props.

 

[Scott06]

Boat is kept in a wet slip. This change occurred while we were at anchor for the day.

Bottom is clean and has ablative anti-fouling paint.

I don't recall hitting anything on our way out. No vibration, but I did not trim drive all the way up and inspect props.

A 2000 would have a replaceable hub prop - I dont think they tend to slip like the older pressed rubber hubs

Would verify the prop is ok and hub not slipping, also run with hatch open to see if there are signs of coupler slipping.

 

[flashback]

Bravo 3 is direct to shaft. No hub.

Really, learn something every day. I thought all props on I/o,s have a hub

 

[muskyfins]

A 2000 would have a replaceable hub prop - I dont think they tend to slip like the older pressed rubber hubs

Would verify the prop is ok and hub not slipping, also run with hatch open to see if there are signs of coupler slipping.

Bravo 3 does not have a replaceable hub prop. The props are directly splined to the shaft.

 

[Scott06]

Bravo 3 does not have a replaceable hub prop. The props are directly splined to the shaft.

Didn't catch that you are correct. If RPMS are verified then you are down to prop damage or coupler

 

[Bt Doctur]

What outdrive oil are you using?

 

[dubs283]

Trim limit may be off and getting a slight cavitation at upper limit

 

[tpenfield]

Things to check/verify . . .

Tachometer giving correct RPM reading (does the engine really sound like it is revving to 5K ?)

Outdrive trim - is the trim reading (sender/gauge) off/inaccurate?

Bravo 3 prop blades bent, dinged, etc ? (and yes - no hubs on a Bravo 3)

Clutch?

 

[muskyfins]

What outdrive oil are you using?

I have been using Lucas 75-90 full synthetic gear oil for many years. Change it every year. Never had a problem in any of my boats. This lower is on it's 3rd year. Was a brand new factory Mercruiser unit.

 

[muskyfins]

Things to check/verify . . .

Tachometer giving correct RPM reading (does the engine really sound like it is revving to 5K ?)

Outdrive trim - is the trim reading (sender/gauge) off/inaccurate?

Bravo 3 prop blades bent, dinged, etc ? (and yes - no hubs on a Bravo 3)

Clutch?

Engine really sounded like it was revving that high. To the point I didn't want to leave it there long for further evaluation. I'll get a test tach on there soon.

Trim sender has been off a bit, but I don't rely on it. And I was trimmed all the way down/in. Never even touched it. Had I been trimmed up, I would have expected to see more boat speed too.

I don't recall hitting anything on the way out to our anchorage. But it has that sort of feel. Engine wasn't laboring as if there is less work being done.

I recall hearing about cone clutches slipping against the gear seat. Although I have never experienced or witnessed this.

 

[muskyfins]

UPDATE: Went out Saturday, everything seemed normal. Got out to open water, advanced throttle and everything normal. i even took a photo of my tach and speedo to show the iboats community. 4000 rpm. 30 MPH. Clockwork.



About 10 seconds after this photo was taken, my tach jumped to 4600. Bounced back to 4000, then climbed back to 4600 and stayed. There was absolutely no change to engine sound, no misses or surges, etc. Perfectly steady.

Long story short-it seems like a tach problem which I'd much rather be chasing than a coupler or drive issue. I have heard about internal tach corrosion and switching between 4-6-8 positions to clean it out. Only have 2 more weeks before I have to have it in winter storage, so not too much time to trouble shoot under load and I don't think I can replicate at lower rpm. I'll look at it this weekend and report back, but any further diagnostics (including a faulty tach) might have to wait until spring.

 

[mvn]

UPDATE: Went out Saturday, everything seemed normal. Got out to open water, advanced throttle and everything normal. i even took a photo of my tach and speedo to show the iboats community. 4000 rpm. 30 MPH. Clockwork.

View attachment 370561

About 10 seconds after this photo was taken, my tach jumped to 4600. Bounced back to 4000, then climbed back to 4600 and stayed. There was absolutely no change to engine sound, no misses or surges, etc. Perfectly steady.

Long story short-it seems like a tach problem which I'd much rather be chasing than a coupler or drive issue. I have heard about internal tach corrosion and switching between 4-6-8 positions to clean it out. Only have 2 more weeks before I have to have it in winter storage, so not too much time to trouble shoot under load and I don't think I can replicate at lower rpm. I'll look at it this weekend and report back, but any further diagnostics (including a faulty tach) might have to wait until spring.

I recently had to deal with a tach giving false readings. Mine was a Faria. I've found that the cylinder selector switch can get dirty/oxidized. Switching it to other settings can help clean the contacts. Also, mine had a fine adjustment trim pot with an access hole on the backside. If the pot is also dirty, sweep it back and forth a few times and set it to where it should be (I calibrated mine with a scan tool).

Mark

 

[tank1949]

Bravo 3 is direct to shaft. No hub.

Coupler!

 

[markhodges78]

UPDATE: Went out Saturday, everything seemed normal. Got out to open water, advanced throttle and everything normal. i even took a photo of my tach and speedo to show the iboats community. 4000 rpm. 30 MPH. Clockwork.

View attachment 370561

About 10 seconds after this photo was taken, my tach jumped to 4600. Bounced back to 4000, then climbed back to 4600 and stayed. There was absolutely no change to engine sound, no misses or surges, etc. Perfectly steady.

Long story short-it seems like a tach problem which I'd much rather be chasing than a coupler or drive issue. I have heard about internal tach corrosion and switching between 4-6-8 positions to clean it out. Only have 2 more weeks before I have to have it in winter storage, so not too much time to trouble shoot under load and I don't think I can replicate at lower rpm. I'll look at it this weekend and report back, but any further diagnostics (including a faulty tach) might have to wait until spring.

Before you go replacing the tach on the back there is a switch to change the tach for the engine cylinders you have. Use a screwdriver and switch that back and forth a few time sometimes they get dirty and moving them around cleans the contacts..

 

[tank1949]

hook up cheap extra tack near or on engine and compare.

 

[ROY WILLIAMS]

UPDATE: Went out Saturday, everything seemed normal. Got out to open water, advanced throttle and everything normal. i even took a photo of my tach and speedo to show the iboats community. 4000 rpm. 30 MPH. Clockwork.

View attachment 370561
SO the throttle is not moved then the 30 MPH is 4000 RPM THEN 4600RPM SAME 30 MPH ...troubleshoot the tach and wiring .....schematic ......